Evaluation of the rotational speed and carbon source on the biological removal of free cyanide present on gold mine wastewater, using a rotating biological contactor

Wastewater streams generated during the leaching process of precious metals recovery (e.g. gold, silver) contain high concentrations of toxic compounds, being free cyanide (CN⁻) one of the most important among them. Free cyanide is a toxic compound that affects cellular respiration and causes irreversible environment problems. Thus, effective wastewater treatment processes should be applied to reduce CN⁻ without producing other contaminants. This study evaluated the effect of rotational speed and carbon source on the biological removal of CN⁻ present on gold mine wastewater, using a rotating biological contactor (RBC). A factorial design (2³) was developed by triplicate (N = 24) to evaluate: carbon source concentration (commercial sugar cane, 99% sucrose), free cyanide concentration (CN⁻), and rotational speed (ω) on the biological removal of free cyanide (%B.R.). The bioreactor worked on continuous system under the following conditions: V_RBC = 4.2L, F_I = 0.42L/h (gold mine wastewater at 10 or 300 mg CN⁻/L, liquid culture media with or without carbon source, final pH = 10.5 ± 0.5), HRT = 10 h, and ω = 5 or 10 rpm. According to the factorial analysis: free cyanide concentration ([CN⁻]_i), carbon source concentration ([FC]_i), and rotational speed (ω) affected significantly (p < 0.05) during the biological removal of CN⁻. Furthermore, the highest biological removal percentage (96.89%) was achieved when the RBC worked under the following conditions: [CN⁻]_i = 300 mg/L, [FC]_i = 3.8 g/L, and ω = 5 rpm; even though dissolved oxygen reached levels below 2 mg/L. Thus, biological removal increased when free cyanide and carbon source concentrations were at their highest level 300 mg/L and 3.8 g/L, respectively; however rotational speed (10 rpm) diminished the bioprocess.